As known, the “Powder Injection Moulding” (PIM) process is an industrially well-consolidated method used to manufacture small and medium-sized objects, e.g., weighing just a few grams, with complex shapes and which are made in very large batches, e.g., thousands of pieces, starting with sinterable metal or ceramic powders.
Such process envisages diverse phases:                high-temperature mixing (around 180° C.) of a ceramic or metal powder with a binding substance, conventionally called “binder”, which usually consists of a polymer;        the working of such mix in a granulator at the temperature of about 180° C., to obtain a fluid material ready to be shaped and conventionally called “feedstock”;        pressure injection of the hot feedstock into a cold mould (at around 40° C.);        removal from the mould of the object shaped this way, conventionally called “green body”;        treatment for the removal of the polymer from the green body, conventionally called “debinding”, and obtaining of a semi-finished product called “brown”, ready for sintering;        sintering of the semi-finished product in different atmospheres and at different temperatures depending on the initial type of powder.        
The initial types of ceramic or metal powders usually have a size below 100 μm and round morphology so as to ensure good flowability, isotropic shrinkage during sintering and excellent surface finish.
The choice of the binder to be added to the powder depends on different factors: the good tensile strength of the green body is required, as is the excellent fluidity and flowability of the feedstock inside the mould, ease of removal of the polymer and last but not least, it is best to use non-toxic polymers or which do not give off toxic vapours at processing temperatures.
From an industrial viewpoint therefore, various polymers or additives are usually used such as stearic acid, polyethylene, polypropylene, polystyrene, polyvinyl alcohol, oleic acid, paraffins, waxes, poly ethyl glycol, poly methyl butyl acrylate, glycerine, and others still.
The mixing of powder and binder normally occurs using double-screw extruders or blade mixers; the aim is to obtain an intimate contact between the polymers and the metal/ceramic powder without introducing air bubbles.
The mix is then granulated in order to obtain the feedstock ready to be introduced into the extrusion machine hopper, which again heats the material to 180° C. and injects the fluid feedstock into the cold mould.
To ensure the good quality of the green body, it is fundamental not to have air bubbles inside the feedstock injected into the mould, which in turn has special openings to eliminate any air.
The green body thus obtained is removed carefully from the mould and is ready to face the critical states of debinding treatment, the purpose of which is to remove the binder without introducing porosity, cracking, bubbles, or dimensional loss, so as to leave a very unstable skeleton of the ceramic or metal powder.
The semi-finished product obtained after debinding has a minimum mechanical consistency and must be moved with great caution until it is introduced into the sintering furnace, where the actual consolidation of the powder takes place with a linear shrinkage normally between 10% and 20% depending on the volumetric content of ceramic or metal powder in the initial feedstock.
The traditional PIM process does however have a number of drawbacks.
To make a sintered product with good mechanical characteristics and low geometric tolerances, in fact, the semi-finished product obtained after debinding must contain a low amount of residual binder and have a high volumetric powder content.
Depending on the binder used, the debinding treatment adopted in the traditional PIM process can be:                of the thermal type, wherein the green objects are heated, in air or protected atmosphere depending on the cases, so as to melt, decompose and then evaporate the binder. This, inconveniently, must be done in a very controlled way, to prevent the formation of defects such as cracks, bubbles and/or breakages of the moulded product due to the different behaviours of the ingredients introduced to form the binder. The process normally requires a few hours and, for ceramic parts with thickness of a few centimeters, even a few days;        of the catalytic type, wherein the binder is polyacetal based and is decomposed catalytically using gaseous nitric acid and/or oxalic acid. This process cuts the debinding times and creates less problems as regards bubbles or breakages compared to the previous one but, nevertheless, it has a negative environmental impact and involves high plant costs;        of a chemical type, wherein acetone, hexane and/or toluene are used as solvents, to remove the binder chemically. This is a very effective but hard to use method due to problems involving the environment and the toxicity of the solvents;        of a chemical-physical type, where the means of removal consists in supercritical carbon dioxide at high pressure and temperature, which is in an intermediate state between gaseous and liquid, and is therefore distinguished by low viscosity, thereby minimising treatment time. This method too however has far from negligible costs.        
Besides the above drawbacks tied to the type of debinding used, it should be underlined that the traditional PIM process does not allow obtaining metal or ceramic objects with considerable wall thickness inasmuch as the debinding cycle required to prevent the formation of bubbles and/or cracks would be so long as to make the process uneconomical and in any case critical.
To overcome the problems associated with the debinding treatment and obtain complex products with good mechanical characteristics, it is possible to follow the teachings contained in the U.S. Pat. No. 4,734,237.
This document describes a PIM process that uses water and gelling agent, usually, but not only, a hydrocolloid, to obtain the binder to be added to the ceramic and/or metal powder.
The water works as a solvent while the gelling agent has the property of forming a gel when it is cooled from a temperature of 80° C. to room temperature.
The feedstock is then prepared at temperatures of 80-90° C. mixing the ceramic/metal powder with water, hydrocolloids, dispersing and plasticizing agents to form a mix of treacly consistency.
The feedstock obtained this way is kept at a temperature such as to cause part of the water considered in excess to evaporate and is then introduced into an extruder and injected into the cold mould.
After removal from the mould, the green body thus obtained undergoes a simple drying process and is ready to be sintered.
Further changes are then suggested in the U.S. Pat. Nos. 5,258,155, 5,397,520, EP 0 968 067, EP 1 113 893, U.S. Pat. Nos. 6,262,150, 7,517,400.
Nevertheless, despite the improvements made with the different patents and the practical innovations made in the industrial field, a number of drawbacks still remain that make the PIM process with water-based binder little used.
In particular, the fact is underlined that the tensile strength of the green body obtained by means of water-based binder is much lower compared to a polymer binder, which makes the removal phase from the mould critical and does not allow faithfully maintaining the geometric shape given by the mould.
Furthermore, the capacity of the water-based feedstock to run inside the mould is rather low because, to obtain less linear shrinkage during sintering and, therefore, have fewer distortions, the feedstock must have a high content of metal/ceramic powder rather than water.
To overcome the low feedstock fluidity, plasticizing agents or high hydrocolloid contents are sometimes used which inconveniently prevent the possibility of performing the debinding by means of simple drying.
It should not be forgotten in fact that the water content in the feedstock must inconveniently remain constant during the entire moulding phase to prevent any change in the rheological characteristics of the feedstock and therefore in the mould filling capacity.
To this must be added the fact that the majority of hydrocolloids requires the presence of Na+, Mg2+ or K+ ions to carry out its functions, but such elements are undesired in the finished product due to problems of corrosion as regards the metals and drop in dielectric strength in the ceramics.
All these problems strongly restrict the use of the PIM process with water-based binder, above all as regards low productivity.